As oil wells and gas wells (hereinafter, as a general term of oil wells and gas wells, referred simply to as “oil wells”) become deeper, oil-well steel pipes (hereinafter, referred to as “oil-well pipes”) are required to have higher strength.
To meet this requirement, conventionally, oil-well pipes of 80 ksi class, that is, having a yield stress (hereinafter, abbreviated as “YS”) of 551 to 655 MPa (80 to 95 ksi) or oil-well pipes of 95 ksi class, that is, having a YS of 655 to 758 MPa (95 to 110 ksi) have been used widely. Further, recently, oil-well pipes of 110 ksi class, that is, having a YS of 758 to 862 MPa (110 to 125 ksi), and further oil-well pipes of 125 ksi class, that is, having a YS of 862 to 965 MPa (125 to 140 ksi) have begun to be used.
Further, the oil and gas in most of the deep wells having been developed recently contain corrosive hydrogen sulfide. In such an environment, hydrogen embrittlement called sulfide stress cracking (hereinafter, referred also to as “SSC”) occurs, and resultantly the oil-well pipe is sometimes broken. It is widely known that with the increase in strength of steel, the susceptibility to SSC increases.
Therefore, in developing high-strength oil-well pipes, not only the material design of high-strength steel is required to be made but also the steel is required to have SSC resistance. Especially in developing high-strength oil-well pipes, the prevention of SSC is the biggest problem. The sulfide stress cracking is sometimes referred also to as sulfide stress corrosion cracking (“SSCC”).
As the method for preventing SSC of low-alloy oil-well pipes, methods of (1) high purification of steel, (2) mode control of carbides, and (3) refinement of crystal grains have been known.
Concerning the high purification of steel, for example, Patent Documents 1 and 2 propose methods for improving the SSC resistance by mean of restriction of the sizes of nonmetallic inclusions to specific ones.
Concerning the mode control of carbides, for example, Patent Document 3 discloses a technique in which the ratio of MC-type carbides to total carbides is 8 to 40 mass % in addition to the restriction of the total amount of carbides to 2 to 5 mass % to tremendously improve the SSC resistance.
Concerning the refinement of crystal grains, for example, Patent Document 4 discloses a technique in which the crystal grains are made fine by performing quenching treatment two times or more on a low-alloy steel to improve the SSCC resistance. Patent Document 5 also discloses a technique in which the crystal grains are made fine by the same treatment as that in Patent Document 4 to improve the toughness.
Conventionally, in producing low-alloy steel materials in the field of seamless steel pipes for oil well and the like pipes, to attain strength properties and/or toughness, heat treatment of quenching and tempering has often been performed after the finish of hot rolling such as hot pipe making. As a method for heat treatment of quenching and tempering of the seamless steel pipe for oil well, conventionally, a so-called “reheat quenching process” has generally been performed, in which process, a steel pipe having been hot rolled is reheated in an offline heat treatment furnace to a temperature not lower than the Ac3 transformation point and is quenched, and further is tempered at a temperature not higher than the Ac1 transformation point.
However, in recent years, from the viewpoints of process saving and energy saving, there has also been performed a process in which a steel pipe having been hot rolled is directly quenched from a temperature not lower than the Ar3 transformation point and thereafter is tempered (a so-called “direct quenching process”) or further a process in which a steel pipe having been hot rolled is sequentially soaked (hereinafter, especially referred also to as “supplementarily heated”) at a temperature not lower than the Ar3 transformation point and thereafter is quenched from a temperature not lower than the Ar3 transformation point and thereafter is tempered (a so-called “inline heat treatment process” or “inline quenching process”).
As disclosed in Patent Documents 4 and 5, it has been widely known that a close relationship exists between the prior-austenite grains of low-alloy steel and the SSC resistance and toughness, and the SSC resistance and toughness are decreased remarkably by the coarsening of grains.
In the case where the “direct quenching process” is adopted for the purpose of process saving and energy saving, the prior-austenite grains coarsen, so that it sometimes becomes difficult to produce a seamless steel pipe excellent in toughness and SSC resistance. The above-described “inline heat treatment process” somewhat solves this problem, but is not necessarily comparable to the “reheat quenching process”.
The reason for this is thought to be that in the simple “direct quenching process” and “inline heat treatment process”, in the case where only tempering is performed as the heat treatment of the postprocessing, there does not exist a process of reverse transformation from ferrite of body-centered cubic structure to austenite of face-centered cubic structure.
To solve the above-described problem of coarsening of crystal grains, Patent Documents 6 and 7 propose methods in which a steel pipe having been directly quenched and a steel pipe having been quenched by inline heat treatment, respectively, are reheated and quenched from a temperature not lower than the Ara transformation point before the final tempering treatment.
In Patent Documents 4 and 5, tempering is performed at a temperature not higher than the Ac1 transformation point in between the reheat quenching treatments of plural times, and in Patent Documents 6 and 7, tempering is performed at a temperature not higher than the Ac1 transformation point in between the direct quenching treatment and quenching treatment performed in inline heat treatment, respectively, and the reheat quenching treatment.